5. What is the main difference between frequency tolerance and frequency stability?

Sometimes the "reference" frequency may be referred to the nominal (spec) frequency, if it is so specified by customers. The frequency stability is usually stated in parts per million (ppm). The frequency tolerance of a crystal is defined as the maximum allowable frequency deviation, in ppm, from the nominal (spec) frequency at a specified temperature, usually +25XC (2XC)

6. What happens to the performance of a crystal when it’s not operating within the temperature range stated in the specification?

The crystal performance will be affected. We highly do not recommend such to take place. It can cause the frequency of the crystal to drift. Worse scenario is it may cause malfunction of customer circuit.

7. What is AT or BT Cuts?

Crystal carries, mainly, its "frequency stability" characteristics as a result of how the quartz bars are cut, in a certain pre-oriented angle, into crystal wafers. Today the most popular and widely used one is the AT-Cut.

The AT-cut has a cutting angle of around 35X15’ to the Z-axis in the negative Y-axis direction, as compared to a -45X to the Z-axis in the positive Y-axis direction for the BT-cut. For ease of understanding, a graph of the two cuts are shown below.

Generally the BT cut blanks are thicker than the AT Cut one at the same frequency, so higher frequency can be achieved using BT cut. One major difference between AT-cut and BT-cut is the frequency stability characteristics. Please also refer to the temperature coefficient curves of the two cuts below.

8. What is the aging of the crystal?

Aging is the change in frequency of a crystal over time. Aging can be in the positive or negative direction. Aging effect contributes to the overall frequency drift of the oscillator that the crystal is used in. Aging will be mainly affected by two important factors, namely, contamination and stress. Experiment proves that contamination on crystal wafer usually causes a negative frequency shift, whereas excessive stress often results in positive frequency drift. When crystal wafer is mounted on a holder (base), it could very possibly be pushed, pulled, or twisted by the mounting structure. This causes stress on the crystal wafer. Such stress will be released or relaxed with time and thus results in positive frequency shift. When assembling the crystal unit, proper mounting method of wafer and using the proper holder (base) will help eliminate or reduce the unwanted stress. For a finished crystal, thermal cycling can be used to expedite the process of exercising & relaxing the mounting stress. Contamination on wafer could happen in various stages of crystal production. Contamination that attaches itself to the surface of wafer causes negative frequency shift because of mass loading effect. Contamination should be minimized by improving cleanliness of manufacturing process as well as cleanliness of wafer in every production step. Crystals can be "pre-aged", to a certain degree, to minimize the effects of aging. Because aging characteristics tend to follow a logarithmic curve, most of the aging of a crystal will occur in the first year of its life. The rate of change of crystal frequency is relatively more or faster during the first year than the second year and beyond.

9. What is pull-ability?

The pull-ability of a crystal is a measure of frequency change as a function of load capacitance. Circuit designer can accomplish an operating frequency range by changing or varying the load capacitance of the crystal. The operating frequency range is determined by the pull-ability of the crystal at a given (varying) range of the load capacitance.

10. What are spurious frequencies?

It is possible for a crystal to vibrate at frequencies that are not related to its fundamental nor overtone frequencies. Such unwanted frequencies are referred to as spurious.

Effects of spurious frequencies can be suppressed in the crystal design & manufacturing stage by changing crystal wafer size, electrode pattern design, and adjustment of metalization on crystal wafer.